feat(cli): add gateway backend selection

[drew]

Summary

Add a --backend selector to openshell gateway start while keeping k3s as the compatibility default.
This adds host-run kubernetes and vm gateway backends, preserves the existing k3s bootstrap path, and makes gateway lifecycle commands backend-aware.

Related Issue

N/A

Changes

• add --backend k3s|kubernetes|vm|podman to openshell gateway start, defaulting to k3s
• keep the current Docker bootstrap implementation behind the k3s backend and pin mise run cluster to it explicitly
• add host-run gateway startup for kubernetes and vm, with XDG-managed PID, log, TLS, and database state
• store backend metadata and use it to dispatch gateway stop, gateway destroy, doctor logs, and doctor exec
• enforce localhost-only gateway destroy targets and return a clear unimplemented error for podman
• prefer OPENSHELL_GATEWAY_HOST for cluster bootstrap host overrides and document it in .env.example

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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